Loss of air pressure key to Helios plane crash

Crash investigators are continuing their probe into the Greek passenger jet disaster just north of Athens on Sunday, with the early evidence pointing towards a sudden and catastrophic loss of cabin pressure at high altitude.

The Helios Airways Boeing 737 crashed about 25 miles north of Athens International Airport at 1203 local time killing all 121 people on board. Radio contact with the plane was lost at around 1030, as it flew high above the Aegean Sea.

The pilots of two Greek fighter jets scrambled to intercept the 737 escorted it for 42 minutes. They saw oxygen masks dangling from the roof of the passenger cabin and observed the co-pilot, alone in the cockpit, slumped over the controls. During another close fly-by, the fighter pilots saw a passenger or steward wrestling with the plane’s controls.

Their observations suggest the plane experienced a dramatic loss of cabin pressure. Pressure inside a 737 is maintained at the equivalent of 3000 metres (10,000 feet) altitude by a mechanical system of valves. A sudden loss of pressure may be caused by a mechanical failure to this system or a breach to the hull itself.

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The 737 was cruising at 9800 m (32,000 feet) when radio contact was lost. At this altitude, a total loss of cabin pressure would cause the temperature inside the plane to drop dramatically, to between -40°C and -60°C, and the air would rapidly become unbreathable as oxygen levels suddenly dropped.

No distress call

But a simple alarm system that monitors air pressure and automatically deploys oxygen masks inside the passenger cabin and the cockpit should immediately have alerted the pilots to the situation.

The pilots are trained to don an oxygen mask on as quickly as possible and quickly bring the plane down to an altitude of around 3000 m (10,000 feet). At that altitude, the air is breathable and much warmer. The pilots should then issue a distress signal and make an emergency landing at the nearest airstrip.

Instead of following this course, the Helios 737, en route from Cyprus to Prague via Athens, cruised on autopilot for more than an hour, before crashing into the ground near the town of Grammatikos, on the southern coast of Greece.

Reported problem

Kieran Daly, at Air Transport Intelligence magazine says the pilots would have had just seconds to react to a rapid loss of pressure at such high altitude. “The point is, especially at that altitude, loss of consciousness happens very quickly – perhaps in 20 seconds,” he told New Scientist. “If you do not react instantly, it can be too late.”

Unconfirmed reports suggest the pilot of the 737 highlighted a problem with the plane’s air conditioning system in a radio message sent just a few minutes before contact was lost. The pilot is reported to have then radioed again to say the problem had been fixed before taking the plane to its cruising altitude.

As the air conditioning system is linked to the one that maintains cabin pressure, this problem could have been linked to the loss of pressure. “Minor problems have a tendency to develop into catastrophic problems, and I think that’s what happened,” says Chris Yates, an aviation analyst at Jane’s Transport Group. He adds that the pilot may have been away from his seat when the loss of pressure occurred, and the co-pilot may not have put on his oxygen mask quickly enough.

Voice recorder

Investigators picking through debris at the scene of the crash have recovered the flight data recorder, which should reveal precisely what happened to air pressure aboard the plane. The cockpit voice recorder was not immediately recovered but, as this only records 30 minutes of sound prior to impact, it is unclear whether this will prove useful to the investigation.

Shortly after the crash, Greek government officials said many of the plane’s passengers were frozen solid when it crashed. However, autopsies carried out on Monday suggest many people were alive, although not necessarily conscious, when the plane hit the ground.

The situation was confused further by reports that one passenger had sent a text message from the plane, in which the pilot was said to have turned blue. On Monday, a Greek man who claimed to have received the message admitted it was a hoax and was arrested by police.

Further unconfirmed reports from a Cypriot newspaper indicate that the crashed Boeing 737 experienced a previous decompression problem during a flight in December 2004.

On Tuesday, another passenger plane crashed, this time in a remote region of western Venezuela, killing all 160 people aboard. The MD-82 craft was travelling from Tocumen International Airport south of Panama City, to Martinique, when the pilots reported problems with both engines and requested permission to descend from 9400 m (31,000 feet) to 4,200 m (14,000 feet). Radio contact was lost at 0300 local time.